Apolipoprotein B mRNA editing enzyme, catalytic polypeptide-1 like 3G (APOBEC3G, A3G) corresponds to a host-derived cytidine deaminase that displays potent anti-retroviral activity. When incorporated into budding HIV virions, the A3G enzyme massively mutates nascent HIV DNA produced during reverse transcription in the next target cell thereby halting HIV growth. HIV counters these effects of A3G through its Vif gene product, which promotes accelerated proteasome-mediated degradation and partially impaired de novo synthesis of A3G. The intracellular depletion of A3G makes the antiviral enzyme unavailable for incorporation into progeny virions. Our recent studies have unveiled a second antiviral action of A3G operating in resting CD4 T-cells. In these T-lymphocytes, cellular A3G functions as a highly active post-entry restriction factor blocking the growth of both wild type and deltaVif forms of HIV. Whether this "Vif-resistant" anti-HIV defense mediated by A3G involves cytidine deamination or a different mechanism is currently unknown. Further, the mechanism by which this post-entry restricting function of A3G is forfeited when T-cells are activated remains incompletely understood. Similarly, little is known about how host cells safeguard their own DNA from the mutagenic effects of A3G. Finally, it remains unknown whether A3G exerts other key functions beyond these antiviral effects. In Specific Aim 1, experiments will be performed to decipher how A3G and the closely related A3F and A3B antiviral enzymes are regulated in cells. In Specific Aim 2, the mechanism of A3G action as a post-entry restriction factor in resting CD4 T-cells, the range of viruses affected by this restriction, and potential similar functions of A3F will be delineated. Finally, in Specific Aim 3, studies will be conducted to assess whether A3G mediates important non-antiviral functions in mammalian cells. These experiments will involve the preparation and analysis of mice lacking the functional analogue of the A3G gene. Together, this program of proposed experimentation promises to enrich our understanding of the biology of A3G as well as the related A3F and A3B enzymes. With such understanding, new therapeutic strategies for inhibiting HIV growth could emerge.